Preparation of rho-cymene from a mono-cyclic terpene



Patented Nov. 6, 1945 UNITED STATES PATEN r OFFICE PREPARATION or p- CYMENE mom a MONO-CYCLIC TERPENE Washington Hull, Noroton Heights, Conn., assign-Q or to American Cyanamid Company, New York,

N. Y., a corporation of Maine No Drawing. 7 Original application January 9,

1943, Serial No. 411,878. plication December 11, 1943, Serial No. 513,990

3 Claims.

This invention relates to the catalytic dehydrogenation of terpenes. Moreparticularly, the invention, relates to the production of p-cymene by the vapor phase dehydrogenation of such monocyclic terpenes as dipentene, limonene, terpinolene, terpinenes and the like in the presence ,of a suitable catalyst containing oxides of chromium and tin. The present invention constitutes a division or my co-pending application, Serial No. 471,878, filed January 9, 1943.

Due to the growing importance of cymene as an intermediate in the production of such styrenes as methyl and e-4-dimethy1 styrenes, and as an intermediate in the preparations of nitrogenous gasoline additives, increasing interest has been shown in its preparation from terpenes.

Divided and this apthese catalysts are too inactive and the conversions obtained are too low towserve as a suitable basis for commercial development.

When attempts were made' to use-the higher operating temperatures with these catalysts, the

final results were poor. While it' might be generally supposed that the conversion to cymene could be improved by raising the reaction temperature, actually with most catalysts the extent to which cracking occurs is found to increase even more rapidly. An examinatibn of the illustrative equation set forth above shows that the reaction The possibility -of converting a monocyclic terp'ene such as dipentene to p-cymene according to the theoretical equation has long been considered as a method of obtaining the latter on a commercial scale. However,

attention. Unfortunately, while the process is desirable from a procedural point of view, its pracis not simple. Not-only is it necessary to remove hydrogen from some of the carbon molecules but it must also be added to others. Hence the reaction has sometimes been referred to as involv-' ing both dehydrogenation andhydrogenation.

The reaction is further complicated by the possibility of cracking-both the original terpenes and the wholly or partially dehydrogenated reaction products. Therefore, it is not particularly surprising to find that raising the temperature tice has been subject to a number of drawbacks I which in the past have preventedits commercial development.

The principal dimculty encountered in utilizin the process has been in the development of a suitablecatalyst. The reaction has been carried out with some degree of success making use of such catalysts as fuller's earth, kieselguhr, activated carbon and the like. The best results with these 7 catalysts have been obtained. by contacting the terpene with the catalyst at reaction temperatures of, 100-200 C. in the liquid phase and from about 225-350? C. in the vapor phase. However;

with the catalysts proposed by the art actually reduced the cymene production whereas the production of such side reaction products as toluene, ethyl toluene, menthane and the like was increased. In some cases considerable isoprene is found. As a result, it was thought that temperatures above about 325 C. could not, be safely used.

Nor did attempts to make use of more active I types of dehydrogenation catalysts provide ,an acceptable solution to the diiliculty. Metallic catalysts, such as nickel or copper for example,

are too active. 7 In order to prevent excessive cracking and tar formation they must be used at such low temperaturesin producing p-cymene that the equilibrium conditions are unfavorable. Metallic catalysts of these types as well as the easily reducible metal oxides are particularly troublesome in that they tend to crack ofl the isopropyl group. Many catalysts were found to produce adisproportionate yield of p-menthane. This latter is particularly unfortunate since once the production of p-menthane in appreciable quantities occurs, the equilibrium conditions are such that it is exceedingly diflicult to completely dehydrogenate to cymene unles excessive contact times are employed. This latter-is impractical since it tends to increase the losses of desirable products by increasing the amount of cracking which occurs.

' Based on'these experiences the art believed that it was necessary to find a catalyst which I would not only give agcod conversion without a disproportionate yield of p=menthane, but that it must also find a catalyst which woind be operative at temperatures low enough to prevent cracking dimcultles. One process, developed by Q research along these lines, is set forth in my I]. 5.

Patent 2,272,711. In that case it was shown that These catalysts, while giving excellent results are subject to a number of inherent drawbacks from the point or view or commercial development. The catalyst itself is, from an industrial [point of view, relatively expensive to prepare.

Further, since a number of materials tend to poison the catalyst, it is desirable to feed a ma- -terial of high purity in order to prevent undue shortening oi the active life of the metal catalyst.

comparison of the results obtained when using difierent operating conditions and difiierent cate= lysts to be made.

In carrying out the process of the present invention, the monocyclic terpenes which constitute the raw materials may be obtained from any suitable source. Dipentene, which is commerciaily available in acceptable quantities, was

In addition, they cannot be readily activated merely by heating in the presence of air, a. procedure which is easily carried out and is desirable in a commercial process.

.Where it is desirable to produce a. product of particularly high purity, the palladium metal catalysts are highly superior. However, from the point of view of commercial productionon a large scale, ease of preparation at low cost and case oi reactivation are such desirable properties in the catalyst to be used that they may often outweigh the added advantage of being able to produce a more nearly pure product without the necessity for further purification.

Consequently, there remains a demand for a process using a suitable catalyst which will give a good cymene yield, low in p-menthane content, and without an undue amount of cracking either of the material fed or of the intermediate products. At the same time, it is highly desirable that the catalyst be one whichis relatively cheap to produce, has a long .active life and can be readily reactivated. It is thereiore an object or the present invention to develop such a group of catalysts and a process by which it can be utilized.

In general the objects of the present invention are accomplished by carrying out the dehydrogenation in the vapor phase using a catalyst comprising chromium oxide with or without the addition or one or more aumliary oxides selected found to give excellent results. It may be obtained for example by isomerization of c-pinene, which is in turn obtained from the sulfate turpentine produced as a by-product in the suliste digestion of coniferous woods for the manufacture or paper pulp. The c-pinene may be isomerized as a separate reaction either in the vapor or liquid phase, and the vapors of the resultant monocyclic terpenes passed directly over the catalyst of thepresent invention. It is not necessary that the monocyclic terpene be pure. Accordingly, the products obtained by the isomerization oi a-pinene for example may be used directly without fractionation.

Nor is the process of the present invention restricted to the use of a particular apparatus. The materials to be treated must be vaporized and the vapors passed. over the catalyst mass in the reaction chamber at a temperature and rate dependent upon the amount of catalytic mam being used. The reaction products may be colleoted as by condensation and separated into their respective components as by fractional distillation. So long as these functions may be performed, the design and exact structure of the apparatus may be varied also at will. The development work was carried out-using both glass and stainless steel reaction vessels. However,

. any material which is catalytically inactive, does not contaminate the materials and is resistant to intergranular attack by the hydrogen liberated during the reaction may be used.

Two controls were found to be of primary importance in carrying out the reaction successfully.

These are the temperatures at which the reaction is carried out and the rate at which the vapors from the group comprising the oxides of tin, sup-' ported on a non-pyrolytic carrier. Contrary to the teachings of the prior art it was found that 7 it was not necessary to keep the reaction temperature below 325 C. Not only is it unnecessary but wholly impractical to do so, since the reaction temperatures of 150-200 0. higher than the limits imposed by the prior art were iound to give the s best results. In this way conversions as high as are passed over the catalyst. While the temperature is the least critical, both are important in obtaining satisfactory results. It was found that the optimum temperature varied with the nature of the feed, the amount of feed, the volume oi catalyst and the like. Once the approximate optimum is determined, variations from this optimum of from 25-30 C. in either direction are not particularly objectionable, although the closer the control the better the results. Too low a temperature results in a poor production, due to the lowering of the reaction rate. Too high a temperature results in excessive cracking and tar formation. In general, temperatures of from about 425-500 C. were found to be satisfactory for a rate of feed within the ranges tested. The invention, however, is not meant to be so limited since in some cases temperatures as high as 525 0., orhigherproved satisfactory.

The rate of feed, vaporized per unit time was found to have the most pronounced eflect on the degree of conversion to the desired products. In general, a feedrate. ofi'rom 10 to cc. per hour per 100 cc. of catalyst was found to give good results within the temperature ranges from 425-500" 0., although the invention is not neces- V sarily to be, so limited.

Mixed'chromium oxide and tin oinde catalysts such as those of the present invention have a particular advantage, in addition to their being relatively inexpensive and easily prepared in a,sss,ss9 tity in the proper size range. This difllculty can that they can be readily reactivated. Further they exhibit little, if any. decrease in catalytic activity after as many as thirty cycles. Regeneration is readily carried out by heating the mass while passing air over it. With some of the catalysts, particularly those containing oxides of tin, it is well to flush out the catalyst with hydroyou after the burning off. However, this is not absolutely necessary since acceptable results may be obtained without so doing.

The method of preparing. the catalyst for use in the process of the present invention may be varied according to circumstances without departing from the scope of the present invention. Preferably it should be done under conditions which promote the lowest apparent density so as to create a large surface area. In this respect the catalysts of the present invention have an advantage over those set forth in my copendin application Serial No. 471,877 filed January 9,

1943 now Patent 2,376,252. In that case the oxide masses themselves were used as the catalyst. In

oxides is more eiflciently utilized.

In addition to the advantage obtained by taking better advantage of the available surface area of the oxides, the catalysts of the present invention have other .marked advantages in that they are generally easier to prepare. .Further the materials used as supports are physically stronger than the oxidemasses themselves so that the resultant mass is easier to handle both in the process of manufacturing the catalyst and in the conversion processes themselves. The supported catalysts exhibit less tendency to settle in the reaction chamber and' thereby interfere with the free flow of vapors. They can withstand higher temperatures without physical damage than can the unsupported oxides, so that quicker and more emcient reactivation can be carried out by burning in air.

In preparing the catalyst so as to obtain the, dd distribution of the oxides on the carrier and maintain the lowest bulk density any desirable procedure may be used. For example, a solution of the metal or metals in. water may be used to impregnate the carrier mass after which it can. be dried and the insoluble metal compound reduced to the corresponding oxide in situ. The carrier y be impregnated either by spraying on the carrier mass or by adding the carrier mass to the solution if the latter is sumcient in volume. Another procedure which can be used to obtain satisfactory results is to impregnate the carrier by using a suspension of the metals in their oxide form. This procedure is ordinarily used where it is diilicult or the facilities are lacking to convert the metal compound to the oxide 2 in place. 7

The physical form in which the finished catalyst occurs may also vary quite considerably.

In some cases a support may be made into granules of a suitable size and the oxides deposited thereon. This is probably the easiest and, cheapest method and is therefore preferable. How: ever, in other cases it is difficult to obtain coherent granules of the carrier in sufllcient quanvessel.

7 'Where difliculty in pelleting the parts the desired coherence to the particles.

be met in any one of several ways. The support may be powdered and made into pellets upon which the oxide massis depositedfor example, or the oxides may be mixed with the powder and the whole made into pellets.

material is caused either by the lack of self-adhesion or by the pellets being difllcult to remove from the mold, a, binder and/or a moldlubricant may be found useful. Substantially any combustible,

wax-like material may be used for this purpose.

Good, results may be obtained using a material such as stearic acid in amounts of 1 to 2%. This material is an excellent mold lubricant and im- It is easily burned out after the pellets are placed in the converter. Where a permanent binder is necessary, water soluble silicates maybe used. Pelleted material is particularly useful in that it enables a uniform packing of the reaction A considerable variation in thenature' and properties of the oxide which goes to make up the catalyst may also be made without departing from the scope of the present invention. The chromium oxide which is used in combination with the tin oxide may be in either the three cipally on the makeup of the other oxides present and operating conditions. presence of chromium, as an omde, in both forms is helpful. In general the chromium oxide should of tin per part of chromium oxide.

be present in from about 1-3 ml parts for from 1-3 mol parts of the oxide or oxides of tin.

The auxiliary oxides used in coniunction with the oxides of chromium may be varied considerably, both in. composition and in amount. In developing the process of the present invention it was found that with the optimum conditions of feed rate and temperature, best results were obtained using from 1-3 mol' r oi the oxides The invention, however, is notnecessarily so limited. Useful results are obtained using proportions ranging from 1-3 moi parts of chromium oxide to about 1-3 parts of the auxiliary metal oxide.

Replacing all or part of the auxiliary metal oxide with an alkaline earth metal oxide such as barium or calcium usually has a slightly beneficial action on the production rate but decreases the yield of cymene by increasing the cracking rate. The presence of uncombinedalkali metal oxides appears to be undesirable in that it prevents the proper "migration" of the hydrogen which must occur in cymene formation.

All of the catalysts of the present invention also have a marked advantage in that they may be used without the necessity for a diluent gas. Many dehydrogenation catalysts, useful for other purposes such for example as metallic nickel, are so active, at the temperatures at which the equilibrium is favorable to cymene production, that the time of contact must be controlledv by means of a diluent. This is highly undesirable since it complicates both the process and apparatus and adds to the cost of operation. In most cases the catalysts of the present invention give better results in the absence of a diluent than they do when such a gas is used.

A number oi non-pyrolytic materials are useful In many cases thelatter, and also as good in use',.are br art. Notable among these are and aluminum oxide. -'l.he activated carboncotalyst used in the prior art is also a. good material-except sctivsted alumina of commerce tlcles of silica bricks. Closely of burnt clay. The'iclsys. however,- should free asposzibleiromolkeline metal ozidessincethese tend to .promoteexcessive dehydrogenation and/or craokilm' Another useful sup rt is 15 titanium dioxide although the letter suflers irom the drawbsck that it must be powdered and pelleted to uniform size in order to serve as support for the other oxides.

I theorems; of materials tried end found useful, activated alumina. is perhaps the most dwireble since it is commercially sveileble in forms resdyfoi" use, is relatively cheap and exhibits the least tendency among the sul lwrts tried to promote crecking. The invention, however, is not mesnt to be so limited since as pointed out good results were obtained using silica. gel granules, broken burnt clay particles, titanium dioxide and kleselguhr es supports.

The invention will be more fully explained in connection with the following examples which are mount to be illustretive only end not by way of limitation.

In the following examples, the reset-ion products were enelyeed to find the degree of conversion by passing dry Hinges through 25 cc. sam= ples of the-condensate in on ice both at the rate of shout 2 bubbles per second for 1%, hours. The ice both was then removed and the dos bubbled through et the same rate for 1 hour at room temperature. Free HCl was removed by evacuating at 10 mm. of mercury for V2 hour at room tempersture. Amoroxlmotely 2 guns. of the sample was then treated with 59 cc. of neutrel methanolefter which to cc. of stendsrdized elooholic KOH was added end the mixture refluxed for 2 hours. Since-the unconverted tomes iorm stable bynot,l-titrotiouot the some KOH utter soponificot ifin gl v% 1:13 of Juli-r2231: lathe condensate.

r A solution-of are m. oifCrQs mam oi 55 water to which was added cc. of concentrated ,200 cc. of wstsr. The resultant solution was sprayed onto 300 cc. oi 0 to 10' mesh granules of 70 burnt clay refuse while the mass was heated gently. 120 cc. of the resultant product was,

' packed in an upright column and vaporized dipentene owed thereover st difierent temperatures and reed rstes. Reaction products were 76 r rides whereesthe'desirsble products do so I tion. A'number oi these srethe some materials which were used as catalysts in the-earlier prior collected by condensation and analysed. mustrstive results ere shown in Table l.

sumclent amount oi hot water. To one-half of this solution is added a. solution prepared by dis-- solving 22.1 ems. oi BnClvfiHaO in 150 cc. of nitric acid. This solution was used to impregnate 300 cc. of 8 to is mesh silica gel obtained by' crumblins o commercisl sil-o-cel" brick. Using this materiel es s cstelyst. the best results were obtsined at s feed rate of 10 cc. per hour of '61- i Food,

Temp. (em), 0. ca/km Index West genation oi dipentene to momenta, the following illustrative results were obteined.

Tobie III 'lemp.(ev.),90. Anson Me:

10 15 a5 10 B8 7 10 so .72 5 so, 14 87 1s A solution of 4.8 ms.

81(08): in 200 cc. of

0 water to which 10 cc. oi 80% nitric acid had own added was mixed with a. solution of 33.7. ms. of

SnCh-SHaOln cc. oi water. To the resultant mixture who added 70 co. oi a. CNNOI)! solution co'ntainln: 0.0885 3m. oi CrsO: per cc. and the resultant mixture sprayed onto 800 cc. of 0 to 10 mesh activated alumina. lit cc.oi this catalyst was placed in a vertical converter chamber and heated to 400 C.'whlle passing Isseous nitrogen therethrough. This was followed by hydrogen which was passed through for several hours and finally thespparetus wasugsin flushed out with i nitrogen :ss- A number of runs were made pwe in: vaporized dipentene' over the ootolyst. 'Iypiccl results are shown-in TobleIY.

' on a vacuum iilter.

water and mixed with enough 4 to 8 mesh actimaze iv a 1 Feed, Av.conv., Temp. (an), 0. mm Want Index 10 84 67 10 85 74 10 84 73 10 at 70 26 88 7t To a solution cont ining 50.6 mm. of

' Fe(N:):-9Hz0 in 100 cc. of water was mixed a solution of 34.9 of Such-&0 in 50 cc. of water. 38.2 cc. of a CMNOsh solution containing 0.0995 gm. oi Cr'zO: per cc, with constant stirring was added. The clear cold solution was sprayed onto 300 cc. of d to mesh activated alumina. Typical results obtained in using 105 cc. of the catalyst in a vertical converter for dehydrogenating dl-- pentene vapors are shown in Table V.

and 12.5 gm. moles of commercial sodium bichromate (NaaCraO-z-2H2O) were dissolved in 30 liters of water and approximately 50 gm. moles oi ammonia in the form of commercial aqueous ammonia were slowly added. The exact end point,

was reached when the addition of a few drops of ammonia solution to a filter sample of the solution gave no precipitate. The red-brown precipitate of Cu(OH)NH4CrO4 was collected and washed 7 The wet paste was then thinned to the wnsistency of a. heavy lubricating oil by the addition of a small volume of distilled rated aluminato absorb all of the paste. This mixture was dried overnight at 125 C. and the resultant granules heated in small portions over a free flame. The application of heat was adjusted to maintain temperatures of about 300-4346 C. The resulting black granules consistedoi a coatingof ClaOa in admixture with copper oxide in the ratio Cr'a0::2CuO coated upon and impregnated into the activated alumina carrier. 100 cc. of these granules were used in the dehydrogenation of dipentene as in the preceding examples. Illustrative results are shown in Table VI.

Table w a Temp. (av.),0. g g2 mi Index 20 I 85 20. B9 25 78 25 84 1G 40 83 76 5O 83 77 I claim: 1. A process of producing p-cymene which comprises the steps of maintaining a catalyst body at a temperature of from about 375-500 C.,

completely vaporizing a monocyclic terpene hav inga single isopropyl side chain at a rate of from 0.1-1.0 part or liquid terpene per part of catalyst by volume, passing. the completely vaporized terpene over the heated catalyst, whereby a major portion of the terpene is converted to p-cymene, condensing the condensable portion of the reaction vapors and isolating the p-cymene content therefrom; said catalyst body comprising, 1-3 incl parts otja substance selected from the group consisting of C1101,- CrOa and mixtures of the same; together with from 1 to 3 molparts of tin oxide, supportedona carrier having-ahigh suriaceactivity by exhibiting substantially no tendency to crack oi! the isopropyl side chain at an operating temperature.

2. A process according to claim 1 characterized in that the carrier constitutes about 6 to 10 mesh activated alumina."

3. A process according to claim 1 characterized in that a part of the tin oxide is replaced by a substanceselected from the group consisting o! the oxides of barium and iron.

. WASHINGTON HULL. 

